Over the last few decades, telecommunications carriers have been considering an inexpensive means of using optical fibers to support access to telecommunications services over a last mile of connection between residential and business customers and a central office of a telecommunications service provider. The greatest bandwidth requirement for telecommunications services for these customers is typically not greater than a couple of hundreds of megabits per second (Mbps). To support this bandwidth requirement, studies have shown that passive optical networks (PON) are the access technology that has attracted the most interest and shown the greatest commercial potential.
Referring to FIG. 1, there is shown an embodiment of a traditional passive optical network (PON) 100 for providing access to telecommunications services over a last mile of connection between customers and a service provider. That is, the traditional PON 100 comprises a telecommunications network 102 comprising an optical line terminal (OLT) 104 (e.g., a central office of a telecommunications service provider) connected to a passive optical splitter/combiner 106 via a pair of optical fibers 110 (i.e., an upstream optical fiber and a downstream optical fiber). The passive optical splitter/combiner 106 is connected to a plurality of optical network units (ONU) 108 via a plurality of pairs of optical fibers 112 (i.e., each pair having an upstream optical fiber and a downstream optical fiber). Each of the plurality of ONUs 108 maintains a connection with one or more customers (not shown) for facilitating telecommunications services between these customers and the telecommunications service provider.
The traditional PON 100 operates such that downstream information from the OLT 104 is broadcast to all of the ONUs 108 through the downstream optical fibers 110 and 112. Each of the ONUs 108 only processes downstream information having an address of the specific ONU 108. Also, in the upstream direction, each of the ONUs 108 is allocated a time slot within which the specific ONU 108 sends information, thereby insuring that the specific ONU 108 has sole access to the upstream optical fiber 110 during its time slot and conflicts with other ONUs 108 are avoided.
A number of technologies have been considered by standards bodies as a transport mechanism for transferring information in the upstream and downstream directions in the traditional PON 100. To date, most of these technologies have been asynchronous in nature, such as, for example, Asynchronous Transfer Mode (ATM) and Ethernet.
In a traditional ATM-based PON (A-PON), information to be transferred is packaged into ATM cells for transmission between the OLT 104 and the ONUs 108. Specifically, the International Telecommunication Union (ITU) has specified the downstream bit rate to be 622.08 Mbps and the upstream bit rate to be 155.52 Mbps. The ITU has also specified that the downstream direction shall support 4×56 ATM cells every frame, and that control and management information shall be sent in an ATM cell at a specific time within the frame. The ITU has further specified that the upstream direction shall support 53 cells, each of 56 bytes, per frame, such that the standardized structure of an ATM cell is modified to carry control and management information.
In a traditional Ethernet-based PON (E-PON), information is transferred in Ethernet packets, whereby each ONU 108 negotiates interface parameters (e.g., bit rate) with the OLT 104, and each ONU 108 is subsequently allowed to transmit information at its line rate when permission is given by the OLT 104.
In traditional A-PON and E-PON networks, the transmission of synchronous data (e.g., voice signals) may produce a jitter problem unless circuit emulation functions are employed in the ONUs 108 to retain the synchronous nature of synchronous data signals as they traverse between nodes in the network 102. Also, in traditional A-PON and E-PON networks, an ONU 108 is only allowed to transmit data when permitted to do so by the OLT 104. Further, in traditional A-PON and E-PON networks, complex algorithms are required for allocating upstream time slots for the ONUs 108.
It should be noted that a synchronous technology has also been considered as a transport mechanism for information transfer in the upstream and downstream directions in the traditional PON 100. That is, the Synchronous Optical Network (SONET) is a synchronous transmission system wherein data is transferred based on a 125 microsecond (μsec) frame. However, transferring data from the plurality of ONUs 108 to the single OLT 104 in the traditional PON 100 based upon the 125 μsec SONET frame is a problem because the traditional PON 100 needs 125 μsec to transmit each frame of data. Thus, the traditional PON 100 must wait 125 μsec from the transmission of a current frame to the transmission of a subsequent frame, which is unacceptable.
In view of the foregoing, it would be desirable to provide a technique for transferring information in a passive optical network which overcomes the above-described inadequacies and shortcomings in an efficient and cost effective manner.